As is well known, any instrument which uses a cathode-ray tube, such as the oscilloscope, requires direct current operating voltages much higher than the direct current potentials usually derived from conventional alternating current line voltages. These high voltages cannot be efficiently and economically attained for a plurality of reasons such as having to incorporate heavily insulated transformer windings, bulky and dangerous capacitors and other objectionable features. In addition, when the cathode-ray tube is a beam penetration type cathode-ray tube wherein the color of light emitted by a phosphor screen is changed by switching an electron accelerating voltage between different levels for a color display system the devariation of the necessary high voltages is even more particular. For example cost, reliability, switching speed and the above-mentioned efficiency must be considered.
Heretofore, voltage switching circuits in a color display system of this type in which the color of light emitted by a phosphor screen is changed by switching an electron accelerating voltage between different levels have included two or more silicon controlled rectifiers (SCR's) which are alternately triggered to switch the acclerating voltage between two different levels. As described in U.S. Pat. No. 3,512,036 which teaches a protective circuit for use with SCR high voltage switching circuits, such circuits connect the SCR's so that both SCR's could conduct simultaneously due to some malfunction. In such a case, the switching is latched-up in which neither of the SCR's can be turned off unless the current is interrupted. Therefore, the use of such a switching circuit along with a protection circuit as described in the above-mentioned patent becomes expensive.
In other prior art power supplies, especially where voltages are in the kilovolt ranges, switching of various voltages is usually difficult in that a plurality of relays, or precautions taken to adequately insulate switches and wiring must be undertaken, are utilized. With relays and switches, switching speeds of up to 200 microseconds are necessary to switch the accelerating voltages between different levels in order to change the color of the light emitted by the cathode-ray tube. These long switching times are not effective in state of the art equipment, another disadvantage of the prior art. Also, these prior art power supplies usually employ transistor "stacks" to do their high voltage switching which magnifies control interface due to the operating voltages of the transistor "stacks". Further, these transistor "stacks" are succeptable to breakdown thereby decreasing reliability.